There has been known an organic electroluminescence device (hereinafter, occasionally referred to as an “organic EL device”) that includes an emitting unit (in which an emitting layer is included) between an anode and a cathode and emits light using exciton energy generated by a recombination of holes and electrons that have been injected into the emitting layer.
As the organic EL device, a phosphorescent organic EL device using a phosphorescent dopant material as a luminescent material has been known. The phosphorescent organic EL device can attain a high luminous efficiency by using a singlet state and a triplet state of an excited state of the phosphorescent dopant material. When holes and electrons are recombined in the emitting layer, it is presumed that singlet excitons and triplet excitons are produced at a rate of 1:3 due to difference in spin multiplicity. Accordingly, the phosphorescent organic EL device can attain a luminous efficiency three to four times as high as that of an organic EL device using a fluorescent material alone.
Patent Literature 1 (International Publication No. WO2003/080760) discloses a compound suitable as a phosphorescent host material for use in combination with a phosphorescent dopant material, in which a nitrogen-containing heterocyclic group is bonded to an aryl carbazoyl group or carbazoyl alkylene group. It is disclosed that an organic EL device capable of being driven at a low voltage and exhibiting a high color purity is obtainable by using the phosphorescent dopant material and this compound in the emitting layer.
However, Patent Literature 1 is silent on lifetime of the organic EL device. In order to use the organic EL device for a light source of an electronic device such as an illumination unit and a display, a long lifetime of the organic EL device is required while a voltage thereof being kept low.